There is a demand for capacitors for use in electronic instruments, such as portable telephones and personal computers, to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for its size and good performance. In the tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the mass of the sintered body or to use a sintered body with an increased the surface area by pulverizing the tantalum powder.
The method of increasing the mass of the sintered body unavoidably causes enlargement of the capacitor shape, and thus, cannot satisfy the requirement for downsizing. On the other hand, in the method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering. Therefore, impregnation of a cathode agent in a later process becomes difficult. As means for solving these problems, a capacitor using a sintered body of powder of a material having a dielectric constant larger than that of tantalum is being studied. Materials having a larger dielectric constant include niobium and titanium.
However, the sintered body produced from these materials is not satisfactory because of its large “specific leakage current value”. Elemental niobium or titanium has a large dielectric constant, and therefore, a capacitor having a large capacitance may be obtained. However, a small “specific leakage current value” is a key point for obtaining a capacitor having good reliability. By evaluating the leakage current value per capacitance, namely, “specific leakage current value”, it can be estimated whether a large capacitance can be obtained in a state where leakage current value is reduced to a practically usable value or less.
The “specific leakage current value” as used herein is defined as a value obtained when a dielectric layer is formed on the surface of a sintered body by electrolytic oxidation, and a leakage current value when a voltage corresponding to 70% of the chemical forming voltage is continuously applied at room temperature for 3 minutes is divided by the product of the chemical forming voltage during electrolytic oxidation and the capacitance. That is,Specific leakage current value=(LC/(C×V))(LC: leakage current value, C: capacitance, and V: forming voltage).
In the case of a sintered body using a tantalum powder, the specific leakage current value obtained from the capacitance and the leakage current value described in the catalogue “CAPACITOR GRADE TANTALUM” of Showa Cabot Super Metal is 1,500 pA/(μF·V) or less. In general, the measured specific leakage current value for guaranteeing this specific leakage current value is said to be from ⅓ to ¼ of the value in the catalogue and is preferably 400 pA/(μF·V) or less.
However, in conventional sintered body capacitors where a niobium powder using elemental niobium or a titanium powder is used, the specific leakage current value is large and exceeds the above-described value. Accordingly, these capacitors are lacking in reliability as a capacitor and are not used in practice.
JP-A-55-157226 (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a method for producing a sintered element for capacitors, where agglomerated powder is molded under pressure into a niobium fine powder having a particle size of 2.0 μm or less, the fine powder is sintered, the molded and sintered body is cut into fine pieces, a lead part is joined thereto, and then these pieces are again sintered. However, this patent publication discloses neither a niobium powder containing antimony nor properties of the capacitor manufactured using this powder.
U.S. Pat. No. 4,084,965 discloses a capacitor manufactured using a niobium powder of 5.1 μm obtained by hydrogenating a niobium ingot and pulverizing it. However, U.S. Pat. No. 4,084,965 discloses neither a niobium powder containing antimony nor properties of the capacitor manufactured using this powder.
JP-A-10-242004 discloses a technique of partially nitriding niobium, thereby improving the leakage current value. However, JP-A-10-242004 discloses neither a niobium powder containing antimony nor properties of the capacitor manufactured using this powder.